1. Field of the Invention
The invention relates to a method and apparatus for recovering data when a readback error is detected in a direct access storage device (DASD).
2. Description of the Prior Art
Computers often include auxiliary memory storage units having media on which data can be written and from which data can be read for later use. Disk drive units incorporating stacked, commonly rotated rigid magnetic disks are used for storage of data in magnetic form on the disk surfaces. Data are recorded in concentric, radially spaced data information tracks arrayed on the surfaces of the disks. Transducer heads driven in a path toward and away from the drive axis write data to the disks and read data from the disks. Various data recovery procedures (DRPs) are known in the art for use when an error is detected in the readback of data from the disks.
For example, a side erase data recovery procedure (DRP) is disclosed by U.S. Pat. No. 4,516,165 issued May 17, 1985 to E. A. Cunningham and D. C. Palmer and assigned to the present assignee. The disclosed DRP is effective against otherwise hard failures due to head sensitivity to the low frequency content of adjacent tracks, or from adjacent track misregistration (TMR). In the disclosed side erase procedure, data from the adjacent track sectors are read, stored, and then the adjacent track sectors are erased either with the read/write head on the centerline or with the read/write head moved closer to the track of interest. This allows reading sectors of the track of interest that were not recoverable using other normal methods. After a successful read, all data of the track of interest and the adjacent sectors are rewritten.
U.S. Pat. No. 4,821,125 issued Apr. 11, 1989 to Christensen et al. and assigned to the present assignee discloses a data recovery procedure including a variable delta-V. Delta-V is the minimum voltage change per unit time which tests signals to discriminate between data and noise. Re-reads are conducted using various incremented values of delta-V in the recovery sequence.
A typical data recovery procedure includes rereads, head shifts and at least one error correcting code (ECC). Each specific hard disk drive normally has one specified list defining a data recovery procedure (DRP) that is considered best for the disk file. However, within one disk drive product, there can be several different failure mechanisms that are dominant. Since each recovery procedure has a different probability of success for each failure mechanism, one given DRP or recovery list is not optimum for specific cases.
Individual recovery actions are more or less effective given the unique characteristics of each disk file and the specific hardware system and environment of use of the disk file. It is desirable to achieve the fastest recovery possible. This requires the most effective recovery actions to be provided early in the recovery procedure given the sequential nature of data recovery procedures.